Obesity is among the most pressing public health problems, with about two-thirds of the US adults overweight or obese. Behavioral lifestyle interventions have been shown to successfully produce clinically significant weight losses and improve health. However, access and adherence to these beneficial programs may be limited by proximity to evidence-based programs and time constraints. Development of new weight control technologies that utilize the Internet may reduce some of these treatment barriers. Internet-based weight control programs have been shown effective in producing weight loss; however, in-person delivery of behavioral weight control produces significantly greater weight loss than online delivery of the same program. Motivational interviewing (MI) has been shown to increase weight loss when delivered as an adjunct to in- person weight loss programs but has not been examined in conjunction with web-based obesity treatment. Therefore, the overall goal of this project is to determine if the addition of on-line MI to a web-based group behavioral obesity treatment program will augment weight loss outcomes. A secondary, exploratory aim is to evaluate the most cost-effective approach for adding MI to online obesity treatment by examining delivery of individual MI sessions in both a fixed schedule and a stepped-care, contingent fashion. This randomized controlled trial is designed to determine whether weight losses in a group behavioral weight loss intervention conducted over the Internet are increased with the inclusion of individual, online MI sessions. All participants will receive the same online 6-month group-based behavioral weight loss program followed by 12-months of weight maintenance. Overweight and obese adults at two study sites (N=639; 27% minority) will randomized to: (1) Internet intervention; 2) Internet intervention with individual MI sessions occurring at three month intervals on a fixed schedule (InternetFixed); or 3) Internet intervention with individual MI sessions delivered contingently if participants miss 2 out of 3 consecutive group chat sessions (InternetContingent). Assessments will be conducted every six months and will include measures of body weight, adherence to treatment, motivational factors, website utilization, and treatment delivery cost. PUBLIC HEALTH RELEVANCE: The goal of this project is to determine if the addition of on-line Motivational Interviewing (MI) to a web-based group behavioral obesity treatment program will increase weight loss. A secondary, exploratory aim is to evaluate the most cost-effective approach for adding MI to online obesity treatment by examining delivery of individual MI sessions in both a fixed schedule and a stepped- care fashion. Overweight and obese adults (N=639; 27% minority) at two study sites (AR and VT) will randomized to: (1) Internet intervention; 2) Internet intervention with individual MI sessions occurring at three month intervals on a fixed schedule; or 3) Internet intervention with individual MI sessions delivered contingently if participants miss group chat sessions. Participants in all three conditions will receive the same online 18-month group-based behavioral weight loss program. Assessments will be conducted every six months and will include measures of body weight, adherence to treatment, motivational factors, website utilization, and treatment delivery cost.